Multivibrator with linearly variable voltage controlled duty cycle

ABSTRACT

A MULTIVIBRATOR INCLUDING TWO TRANSISTORS AND A TIMING CAPACITOR, CONNECTED BETWEEN THE EMITTERS OF THE TRANSISTORS, WHICH IS CHARGED WITH A CONSTANT CURRENT. THE POTENTIALS AT THE TRANSISTOR BASES ARE ESTABLISHED BY CONTROLLABLE CURRENTS AND DETERMINE THE VOLTAGE TO WHICH THE TIMING CAPACITOR CHARGES. BY CHANGING THE VOLTAGE   TO WHICH THE CONSTANT CURRENT CHARGES THE CAPACITOR, THE DUTY CYCLE OF THE MULTIVIBRATOR IS CHANGED.

.1971 o. H. GRANGAARD, JR

MULTIVIBRATOR WITH LINEARLY VARIABLE VOLTAGE CONTROLLED DUTY CYCLE FiledNov. 13, 1968 CONTROL VOLT/ 6E VOLT. SOU RCE FIG. I

FIG 2 M. -m I J I I b p. I I J I A m g n r. I" W a n I l I I I ATTORNEUnited States Patent 3,566,301 MULTIVIBRATOR WITH LINEARLY VARIABLEVOLTAGE CONTROLLED DUTY CYCLE Orrin H. Grangaard, Jr., St. Paul, Minn.,assignor to Honeywell Inc., Minneapolis, Minn., a corporation ofDelaware Filed Nov. 13, 1968, Ser. No. 775,333 Int. Cl. H03k 3/282 U.S.Cl. 331-113 6 Claims ABSTRACT OF THE DISCLOSURE A multivibratorincluding two transistors and a timing capacitor, connected between theemitters of the transistors, which is charged with a constant current.The potentials at the transistor bases are established by controllablecurrents and determine the voltage to which the timing capacitorcharges. By changing the voltage to which the constant current charges.the capacitor, the duty cycle of the multivibrator is changed.

BACKGROUND OF THE INVENTION The invention pertains to the field of solidstate active element oscillators. More particularly the invention isdirected to a transistorized relaxation oscillator of the multivibratortype.

SUMMARY or sawtooth output voltage waveform is available across thetiming capacitor. The shape of the triangular or sawtooth output dependsupon the duty cycle.

BRIEF DESCRIPTION OF THE DRAWING FIG. 1 is a schematic circuit diagramof a variable symmetry free-running multivibrator; and

FIG. 2 is an output waveform of the multivibrator.

DESCRIPTION OF THE PREFERRED EMBODIMENT The multivibrator of FIG. 1generates a rectangular waveform of fixed frequency and variable dutycycle. Duty cycle is the ratio of on to olf times. The duty cycle iscontrolled linearly between 0 and 100% by an external signal.

The multivibrator includes an external voltage source 10. Source 10 hasfirst and second terminals, the second terminal is connected to areference potential point, for example, ground, and the potential at thefirst terminal with respect to ground is designated V The multivibratoralso includes first and second switching elements 12 and 14. Eachswitching element has control, input and output terminals. A switchingelement conducts current between its input and output terminals when apredetermined potential is applied between its input and controlterminals. As shown in the drawing, switching elements 12 and 14 aretransistors. The control, input, and output terminals are the base,emitter, and collector electrodes, respectively. Resistors 1'6 and 18,each of the same resistance, are connected from the collector terminalsof transistors 12 and 14, respectively, to the voltage V A resistor 20is connected from the collector electrode of transistor 12. to the baseelectrode of tranice sistor 14 and another resistor 22 is connected fromthe collector electrode of transistor 14 to the base electrode oftransistor 12. Resistors 20 and 22 each have the same resistance. Theresistance of resistors 20 and 22 is substantially greater than that ofre-istors 16 and 18. Resistors 24 and 26, each of the same resistance,are connected from the base electrodes of transistors 12 and 14,respectively, to ground. A timing capacitor 28 is connected between theemitter electrodes of transistors 12 and 14. A first constant currentsink 30 and a second constant current sink 32 are connected between theemitter electrodes of transistors 12 and 14, respectively, and ground. Aconstant current I flows into each of the sinks 30 and 32 regardless ofthe potential at the emitter terminals of transistors 12 and 14,respectively. Current sinks 30 and 32 are alike. Current sink 30comprises a transistor 36 having collector, base, and emitter electrodesand a resistor 38 which is connected between the emitter electrode oftransistor 36 and ground. The collector electrode of transistor 36 isconnected to the emitter electrode of transistor 12. The base electrodeof transistor 36 is connected to a source of potential V. Regardless ofthe potential at the collector electrode of transistor 36 asubstantially constant current I flows into the collector electrode andout the emitter electrode. In this sense, circuits 30 and 32 arereferred to as constant current sinks.

A current source 34 provides second and third currents, I and I to thecollector terminals of transistors 14 and 12 respectively. The sum ofthe currents I and I is constant and the ratio of I and I is variable.Current source 34 includes a pair of transistors 40 and 42. A pair ofresistors 44 and 46, each having the same resistance, are connected inseries between the emitter electrodes of transistors 40 and 4.2. Anotherresistor 48 is connected from a source of potential V to a point commonto resistors 44 and 46. A control voltage 50 is connected between thebase electrodes of transistors 40 and 42. The current flowing out of thecollector of transistor 40 is designated I and the current flowing outof the collector of transistor 42 is designated I The current flowinginto the junction point between resistors 44 and 46 is designated I andis constant.

Diodes 37 and 39 are connected across resistors 16 and 18 respectively.The purpose of diodes 37 and 39 is to shunt current around theirassociated resistors. Diode 37 shunts substantially all of the current Iaround resistor 16 when transistor 14 is conducting. Diode 39 shunts thecurrent I around resistor 18 when transistor 12 is conducting.

If diodes 37 and 39 and transistors 40 and 42 are removed the remainingcircuitry describes a standard form of a free-running multivibratorcommonly called a current-coupled, non-saturated multivibrator which hascertain advantages, such as, fast rise and fall times, good rejection ofsupply voltage variations, good frequency stability with temperaturechanges, and positive starting action because of the non-saturatedpositive feedback circuitry employed. Furthermore with this standardmultivibrator, there is available either a square wave voltage output atthe collectors. of transistors 12 and 14 or a triangular voltagewaveform taken across capacitor 28.

In operation the transistors 12 and 14 are alternately conducting andnon-conducting. It will be assumed that resistors 20, 22, 24, and 26 allhave the same resistance R If I =I =0= the potential at the base of theconducting transistor is about Vcc/Z and the potential at the base ofthe non-conducting transistor is (V -2I R )/2, where R is the resistanceof each resistor 16 and 18. The multivibrator has two states, one statewhen transistor 12 is conducting and transistor 14 is non-conducting andanother state when transistor 12 is non-conducting and transistor 14 isconducting. The transition time it takes to go from one state to theother is determined solely by the difference in the two allowablevoltage states at the base electrodes of transistors 12 and 14 and thelinear charging time constant formed by current I and the capacity C ofcapacitor 28. The potential at the emitters of transistors 12 and 14when they are conducting is about equal to the potential at their bases.The difference between the two allowable voltage states is I R Theaddition of currents I and I does not affect the allowable high voltagestate V /2 because of the action of diodes 37 and 39. The allowable lowvoltage state does change however and it becomes The current I isshunted by diode 37. It is assumed that transistor 14 is conducting andthat transistor 12 is nonconducting. The current 1 can equal I or zero.The difference in the base voltage between the high and the low voltagestates is (I R )I R /2. This is assuming that the resistance R is muchless than R The change in voltage which occurs across capacitor 28 whenthe circuit changes from one state to the other is 2I R I R It is seen,therefore, that the voltage through which capacitor 28 is linearlydischarged and charged by current I is a linear function of I or Idepending upon which transistor 14 or 12, is conducting. The applicationof control voltage 50 linearly varies the ratio of currents I and I I +I=I =a constant. The control voltage 50 which is differentially appliedto the bases of transistors 40 and 42 is therefore linearly controllingthe duty cycle of the multivibrator.

The voltage appearing across capacitor 28 is equal to idt=I t/C where Cis the capacitance of capacitor 28. The time during which transistor 14is conducting is designated t and is equal to R C(2I /I The timetransistor 12 is conducting is designated t and is equal to R C(2I /IThe period of the multivibrator is t +t and is equal to R C(4I /I Theduty cycle is t /(t +t and is equal to B(2I A) where B is equal to1/(4-I /I and A is equal to 1/I From these equations it can be seen thatthe period of the circuit is a function of only the circuit constantswhereas the duty cycle is a linear function of current I; which in turnis a linear function of the control voltage '50. The resistors 44 and 46in the emitter circuits of transistors 40 and 42, respectively,establish the sensitivity of the circuit. Integrated circuit technologycan be readily used in implementing this circuit.

The embodiment shown and described is the preferred embodiment but otherembodiments may be improvised by those skilled in the art.

1. A multivibrator with a linearly variable duty cycle, comprising:

a source of voltage having first and second terminals,

the second terminal connected to a potential reference point, the firstterminal at a potential V volts;

first and second switching elements, each having control, input, andoutput terminals, a switching element conducting current between itsinput and output terminals whenever there is applied to its controlterminal a predetermined potential with respect to that at its inputterminal;

first and second impedances each of the same impedance Z ohms connectedfrom the output terminals of the first and PQO Q S iiQ S m 4 spectivelyto the first terminal of the voltage source;

a third impedance of impedance Z ohms connected between the outputterminal of the first switching element and the control terminal of thesecond switching element, Z being substantially greater than Z a fourthimpedance of impedance Z ohms connected between the output terminal ofthe second switching element and the control terminal of the firstswitching element;

fifth and sixth impedances of impedance Z ohms connected between thecontrol terminals of the first and second switching elements,respectively, and the potential reference point;

a timing capacitor of capacitance C connected between the inputterminals of the switching elements;

first and second constant current sinks connected between the inputterminals of the first and second switching elements, respectively, andthe potential reference point, a constant current 1 flowing into thesinks regardless of the potential at the input terminals of theswitching elements;

means for providing second and third currents, I and I to the outputterminals of the second and first switching elements, respectively, thesum 1 -1-1 being constant and the ratio 1 /1 being variable; and,

first and second current shunting means connected across the first andsecond impedances respectively, the first shunting means shuntingsubstantially all the third current 1;, around the first impedance whenthe second switching element is conducting, the second shunting meansshunting the second current I around the second impedance when the firstswitching element is conducting.

2. The circuit of claim 1 wherein the impedances are resistors ofresistance R and R ohms for the impedances Z and Z respectively.

3. The circuit of claim 1 wherein the switching elements are transistorseach having base, emitter, and collector electrodes for the control,input, and output terminals, respectively.

4. The apparatus of claim 1 wherein each of the current sinks comprisesa transistor, a voltage source, V, and a resistor, the collector of thetransistor connected to the input terminal of a switching element, thebase connected to the potential V, and the emitter connected to therefeernce potential through the resistor.

5. The apparatus of claim 1 wherein the current shunting means arediodes.

6. The apparatus of claim 1 wherein the means for providing the currentsI and I comprises a pair of transistors, a pair of equal resistorsconnected in series between the transistor emitters, another resistorconnected from a point common to the series resistors to a voltagepotential V each transistor collector connected to the output terminalof a switching element, and a control voltage source connected betweenthe transistor bases.

References Cited UNITED STATES PATENTS 3,037,172 5/1962 Biard 331-113X3,076,152 1/1963 Biard et al 3311l3 3,249,893, 5/1966 Castellano, Jr.331--113 ROY LAKE, Primary Examiner S. H. GRIMM, Assistant Examiner US.Cl. X.R. 3 1-1 -44

